Let me chime in and repeat what is mentioned above. FIT is the important thing! A more expensive shotgun that doesn't fit your body type as well as the "cheaper" competition won't serve you well.
Shoes are sold in different sizes for a reason. The same holds true for shotguns. I know it sounds strange because your local gun shop doesn't ask you if want a small, medium or, large. In my case I personally like the aesthetics of the Browning O&U's better then the Beretta's but, the Beretta's stocks are cut to fit me while the Brownings are not.
Unless you are going to get fitted for a shotgun and have the stock made to fit your body profile, focus on well known quality brands with a long term reputation for good quality firearms. Then find the one (or series) that fits you best.
Depending on how big your are, strong you are and, what you intend to do with the shotgun, you may find a specific reason to get a barrel length unusually long or short. In general most people will get a 26 or 28 inch barrel length. If the barrels are light, consider the 28 inch. A little more weight out front will slow your swing down a little but, will help your track and steady the shotgun.
Brand and specifics? In new shotguns, the Beretta and Browning offerings are the way to go. Well known companies with a long and well earned reputation for quality. If you are new to O&U's, stick to the $1K to $2K price range until you really understand what you are buying. That price range offers a lot of value for what you spend. If you have a reputable dealer or very knowledgable friend to help, a used shotgun is a viable option as well. If you don't know what to look for, don't buy one off the street (gun shows, newspaper ads, etc.).
Have you considered a 20 gauge or, are you only looking for a 12 gauge? A 20 gauge can be a significantly lighter weight shotgun if you are going to lug it around the field a lot and shoot it very little. Features to look for? Get a current Browning and Beretta product catalog and educate yourself on the current options for back bored barrels, sport clays features, etc. to ease the buying decision.
In the end, it doesn't matter how much it cost or what features it has if it doesn't fit your body and you can't (or won't) shoot it.